This invention relates to ink jet printers, and, more particularly, to the mounting arrangement of print heads in such printers.
Printers are devices that print characters onto a printing medium such as a sheet of paper or a polyester film. Printers of many types are available, and are commonly controlled by a computer that supplies the images, in the form of text or figures, that are to be printed.
Some printers use a colorant-containing liquid, which may be an ink or a dye, but is generally termed an "ink" in the printer industry, to form the images on the printing medium. (By contrast, other printers use a dry toner to form the image.) Such printers deliver the colorant to the medium using a print head that creates the proper patterning of colorant to permanently record the image.
One important type of printer is the thermal ink jet printer, which forms small droplets of ink that are ejected toward the printing medium in a pattern of dots that forms the images. When viewed at a distance, the collection of dots form the image in much the same manner that images are formed in newspapers. Ink jet printers are fast, produce high quality printing, and are quiet, because there is no mechanical impact during formation of the image, other than the droplets of colorant striking the printing medium.
Typically, an ink jet printer has a large number of individual colorant-ejection nozzles in a print head, supported in a carriage and oriented in a facing, but spaced-apart, relationship to the printing medium. The carriage and supported print head traverse over the surface of the medium, with the nozzles ejecting droplets of colorant, at appropriate times under command of the computer or other controller, to produce a swath of droplets. The droplets strike the medium and then dry to form "dots" that, when viewed together, form one swath or row of the permanently printed image. The carriage is moved an increment in the direction lateral to the traverse (or, alternatively, the printing medium is advanced), and the carriage again traverses the page with the print head operating to deposit another swath. In this manner, the entire pattern of dots that form the image is progressively deposited by the print head during a number of traverses of the page. To achieve the maximum output rate, the printing is preferably bidirectional, with the print head ejecting colorant during traverses from left-to-right and right-to-left.
Color ink jet printers utilize several, typically four, different print heads mounted in the print carriage to produce both primary and secondary colors. Each of the print heads produces a different color, with four often-used colors being cyan, yellow, black, and magenta. These primary colors are produced by depositing a droplet of the required color onto a dot location. Secondary or shaded colors are formed by depositing multiple droplets of different color inks onto the same dot location, with the overprinting of two or more primary colors producing secondary colors according to well established optical principles.
Good print quality is one of the most important considerations and bases of competition in the ink jet printer industry. Since the image is formed of thousands of individual dots, the quality of the image is ultimately dependent upon the quality of each dot, and the arrangement of the dots on the medium. Because of the fashion in which the printing occurs, the quality of the dots can have a surprisingly large effect upon the final image quality, both for black-and-white and color images. The present invention is directed toward improvement of the image by improvements in the quality of the printed dots in color images.
There can be several sources of degradation of the image in color printing, particularly for the bidirectional printing of secondary colors where each dot is produced by overprinting of two primary colors. There can be a perceived color shift due to the different appearance of a droplet of a first color deposited over a droplet of a second color, as compared with a droplet of the second color deposited over a droplet of a first color. That is, a color 1 on color 2 dot usually has a different shade or tint than a color 2 on a color 1 dot. Another shading problem arises when the first deposited droplet has not dried when the second droplet is deposited, causing an intermixing of colors on the medium that creates yet a third shade of color.
Other sources of reduced image quality arise from more mechanical origins. If the droplets from different print heads are not precisely superimposed when a secondary color is printed, causing an absence of registry, the resulting dot usually has regions of three different tints, one for each of the deposited primary colors and an overlap region of the desired secondary color. Mechanical alignment of the print heads to achieve and retain perfect superposition is difficult, for two reasons. One is that the dots are quite small, on the order of a few thousandths of an inch in diameter, and the tolerances on the print heads themselves and their alignment in the carriage are therefore very tight. Further, the print heads must sometimes be changed, as for example when the print head runs dry of colorant, by the user, a person typically not familiar with alignment procedures. The arrangement for supporting the print heads in the carriage must therefore be self-aligning to a high degree of accuracy, even when the alignment is conducted by an unskilled person. Misalignment of dots also arises from backlash of the printer traversing mechanism, and from a directionality effect when a dot is printed with the print head moving from left to right as compared with right to left. Yet another source of mechanical problems in forming the dots is that the relatively large amount of liquid deposited on one location can be absorbed by the printing medium in such a way that the medium becomes irregularly wavy in the vicinity of the dot when the ink dries, a condition known as "cockle".
Existing color ink jet printers produce images of acceptable quality, and are widely used. However, there is a continuing need for improved ink jet printers wherein the dots forming the images are of a reproducible, high quality that is retained in use in a wide variety of printing conditions, even when one or more of the print heads is changed by an unskilled person. The present invention fulfills this need, and further provides related advantages.